Rear Axle Width

Yeah I realize that’s not a great pic, I couldn’t find a good one that showed it more clearly.

I believe the Kart Republic rear hubs might have a 50mm hole all the way through, allowing you to slide the hubs further inward if you wanted, but regardless, modern karts don’t really work that narrow anymore unless you’re looking at a really low HP application or super hard tires.

I just walked outside and checked my KR2. They are a pass through design. The hubs are 97mm total length. If you’re running their wheels at 55 1/8” width you’ll have 65mm of axle inside of each hub and have 32mm left on each hub if you wanted to go to flush with the edge of the hub.


So you’re saying 1/3 of the hub is not supported by the axle? And that makes the difference in the handling?

Correct, more or less. We usually play within a range of 54.5" - 55" for overall rear width, so small adjustments make quite a bit of difference on that.

Regarding the short axle vs. long axle at the same rear width, many drivers would say the shorter axle (less axle in the hub) gives the kart more “dig” mid-corner. It’s a very small difference and could be some placebo effect, but when fine tuning how much the kart is unloading the inside rear wheel, it could make the difference between the kart being flat or hopping.

How that functions, I couldn’t give you a scientific answer. Having less axle in the hub would soften the area within the hub I would think, possibly changing the way the axle flexes on the end only.

If the data doesn’t support the theory, get a new theory (Al Nunley) if you’re lap times show improvement, I may have to change my theory.

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Hi @tjkoyen,
Operating at that width, say 1390 (54.7") and with something like a 95mm long hub and 1000long axle would leave maybe ~50mm of axle into the hub - is this what you see typically?
Is 50mm (1 axle diamater) about the minimum you would run, or do you have substantially longer hubs?

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I always run the OTK medium hub (92mm) and have no issue running rear track at 1390mm on a 1000mm axle. Just always make sure the pinch bolt is over the end of the axle by several mm to prevent it slipping off and you’ll be fine.

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I loved it. This is the reason I login KP daily. Just smart guys discussing meaningful themes with unique point of views.

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Based on what you describe, that a shorter axle allows the inside rear wheel to lift longer, I’d theorise the phenomenon as this: as we turn into a corner, the outside rear hub acts as the fulcrum point for an axle that is lifting the inside wheel.

With less axle contained within that outside hub, less of the potential energy created by jacking the axle can be transferred into the opposite side of the fulcrum point (the axle within that hub), so it remains on the side of the lifted axle (and chassis) to bend it higher and for longer.

…or maybe not. This is just a semi-educated guess. Happy to hear anyone else’s thoughts! Going to try a cut-down axle this weekend me thinks.

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Just wondering. Would going from a 95mm hub to a 85mm hub and keeping the rear width the constant have the same effect as cutting the axle 10mm on each side?

The “spring” energy comes from that portion of the axle that flexes. The portion of straight, unloaded axle (length inside the hub) releases no KE; it’s just along for the ride (pun).

I revive this topic because I’ve read a lot, in this forum and in other sites, about cutting the axle, but something sounds strange. I mean, for what I read, it seems that a shorter axle gives only advantages.

I quote an article by :
" What a shorter axle will provide:

  • Allow the kart to react quicker in the rear, allowing for more rotation
  • Stabilizes kart under braking
  • Holds rear tire up longer
  • Decreases grip on exit"

So the question is, why everybody doesn’t use shorter axle? I think there must be a tradeoff, an advantage of standard axle, because every manufacter sell their karts with 1040mm axle, but I can’t understand where is this tradeoff.

I think another important question is, professional drivers do use shorter axle? I’d like to understand if a shorter axle helps only non professional drivers, covering up their driving problems and therefore it’s counterproductive for a professionist, or if is a common use also in professional teams.

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I think the thing you have to remember is that this is just another tool used for tuning. Every manufacturer also sells their kart with a certain length hub, or specific stiffness axle too, and in most cases the stock/standard setup will work well for most people in most situations. Are there advantages to changing these things…maybe, maybe not. The four bullet points you mention may not be an advantage in every situation.

In my case, I cut the standard axle to use a different brand wheel as they have a different offset than OEM stock wheels and I had zero adjustments for rear width.

The short answer is that it depends on the chassis, tire, driver, and track conditions. All of these factors are going to affect the balance of the kart. With the KR chassis for example, nearly everyone seems to run a cut axle.

At our last race at Road America, I installed a Medium AMV axle which was 1040mm. Next session I cut it down to 1010 and the results were everything you quoted from the article. Biggest difference was the reactivity of the kart, along with quick and more predictable rotation.

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Because sometimes you don’t want the rear to stay up longer or the outside rear wheel to dig harder. It’s a tuning tool, so for example if you are already overloading then outside tire and getting too much inside rear wheel lift, a shorter axle will only make it worse.

It is a small and subtle change between axle length, but I usually use the short axle as my standard setup.

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4 posts were split to a new topic: Birel with a Briggs 206 Hopping in Turns on MG Red

Maybe for KF category, not for KZ, and even for KF I don’t see anyone using short axles, at least not in the European competitions.
Bottom line is, will a shorter axle give you better laptimes?

Yes, professional drivers use shorter axles as well. Actually, I think they are the ones that use it most as it is a fine tunning (in my opinion) and usually weekend drivers do not feel the difference.
When I created the topic below, I didn’t realize there was a topic already discussing this matter, so I described there my opinion about rear axle length.


I’ve been going through this dilemma on my IPK (Praga) recently. The axle that came with the kart second hand is a cut axle of 1020mm. I recently slightly bent it had had to install a new 1030mm axle. For a quick and dirty comparison I found that the rear grip I’d normally have with the 1020mm axle @ 1390mm rear track is about the same as the 1030mm axle @ 1405mm.

Now I’m wondering if I get another axle and cut it back to 1010mm or even 1000mm to see the results. Either that or comparing with short hubs or a less grippy axle…

I’m going to first assume that your end goal is to free the rear of the kart.

Based on my experience with cut axles, I think your need is going to depend on which phase of the corner you’re trying to prioritize. If you’re mainly trying to optimize the last third of the corner (corner exit), then shorter hubs will give you a similar result to a shorter axle. If you are trying to free the rear on corner entry and mid-corner, then a shorter axle is going to be more impactful than shorter hubs, also with the same impact to corner exit described earlier.

Your mileage will vary based on the category you’re racing, as well as your tire compound. I’ve found that as a rough rule of thumb, shorter axles will favor lower HP classes (KA or even X30).